1. Field of the Invention
The present invention relates to a semiconductor laser element, and in particular to a red semiconductor laser element which contains an InGaAlP-based material, and oscillates at a wavelength in the 0.6-μm band.
2. Description of the Related Art
Currently, efforts of developing the InGaAlP-based red semiconductor laser elements which oscillate at the wavelengths of 630 to 680 nm are being actively made for use as light sources in DVD (digital versatile disc) systems, laser pointers, barcode readers, displays, and the like. In particular, in the DVD-R (Recordable)/RW (ReWritable) systems, the data recording rate is required to be increased for handling a great amount of information, and therefore efforts of developing systems which realize higher data recording rates are being actively made. When the data recording rate increases, the length of time to form each pit is required to be decreased. Therefore, the semiconductor lasers used as the light sources are required to have high optical output power and high reliability.
A typical one of the conventional InGaAlP-based red semiconductor laser elements has the following structure. That is, an n-type InGaAlP cladding layer, an undoped InGaAlP optical waveguide layer, a multiple-quantum-well active layer, an InGaAlP optical waveguide layer, a p-type InGaAlP cladding layer, a p-type InGaP heterobuffer layer, and a p-type GaAs cap layer are formed in this order on an n-type GaAs substrate having a principal plane the crystal orientation of which is tilted 5 to 15 degrees from (100) toward (111). The p-type GaAs cap layer, the p-type InGaP heterobuffer layer, and the p-type InGaAlP cladding layer forms a ridge, and the spaces on both sides of the ridge are filled with an n-type GaAs current-blocking layer. In addition, a p-type GaAs contact layer is formed on the p-type GaAs cap layer and the n-type GaAs current-blocking layer. Further, a p-side electrode is formed on the contact layer, and an n-side electrode is formed on the back surface of the substrate.
In most of the conventional semiconductor laser elements, the total thickness of the active layer and the optical waveguide layers is 0.18 to 0.28 micrometers, and a great amount of light generated in the active layer leaks into the cladding layers. Therefore, the thickness of each cladding layer is increased to 1 to 1.2 micrometers so as to prevent loss of the light.
The structure of the conventional semiconductor laser element described above has the following problems (1) and (2) since the cladding layers are made of InGaAlP.
(1) Since InGaAlP exhibits higher specific resistance and higher thermal resistance than AlGaAs, the operating voltage of and the heat generation in each semiconductor laser element increase with the thicknesses of the cladding layers. That is, the increase in the thicknesses of the InGaAlP cladding layers adversely affects the characteristics of the semiconductor laser element.
(2) Since the diffusion coefficient of Zn as a p-type dopant in the InGaAlP materials is great, Zn greatly diffuses into crystals during growth or heat treatment of the crystals, and the diffused Zn intrudes into the active layer and produce non-radiative recombination centers in the active layer, so that the characteristics of the semiconductor laser element deteriorate.
In order to solve the problem (1), Japanese Unexamined Patent Publication No. 5(1993)-243669 discloses the following structure for efficiently confining light.
In the disclosed structure, one or two optical waveguide layers are asymmetrically arranged with respect to an active layer (e.g., an optical waveguide layer is arranged on only the n side of the active layer, or the optical waveguide layer on the n side of the active layer has a greater thickness than the optical waveguide layer on the p side of the active layer), and a high-refractive-index layer having a similar refractive index to the average refractive index of a waveguide layer constituted by the active layer and the one or two optical waveguide layers is arranged in the cladding layer on the n side of the active layer. Since the total thickness of the one or two cladding layers can be reduced in the above structure, the electric resistance and the heat resistance of the semiconductor laser element can be suppressed.
On the other hand, in order to solve the problem (2), U.S. Pat. No. 6,798,808 discloses a structure of a semiconductor laser element for preventing the diffusion of Zn. In the structure disclosed in U.S. Pat. No. 6,798,808, an undoped layer or a layer having a small carrier concentration is arranged as a portion of a cladding layer in a vicinity of an active layer.
However, when the cladding layer on the p side of the active layer is eliminated or the thickness of the cladding layer on the p side of the active layer is reduced as disclosed in Japanese Unexamined Patent Publication No. 5(1993)-243669, the influence of the diffusion of Zn (the problem (2)) becomes more serious. On the other hand, when an undoped layer or a layer having a small carrier concentration is arranged as a portion of a cladding layer in a vicinity of an active layer as disclosed in U.S. Pat. No. 6,798,808, the temperature characteristics of the semiconductor laser element deteriorate with decrease in the carrier concentration in the cladding layer since the carriers in the above portion of the cladding layer in the vicinity of the active layer raise the Fermi level and substantially strengthen the confinement of carriers in the active layer.